Islands in the mist: Urbanisation is punching holes in north India’s winter fog

With land surface temperatures over Delhi recorded at around four to five degrees Celsius higher compared to its rural surroundings, researchers said the formation of urban heat islands burns the base of the fog layer during mid-morning.

Satellite image of the fog holes over the Indo-Gangetic Plain, including New Delhi.

The high level of local pollution is burning the fog over cities in the Indo-Gangetic Plains (IGP), with Delhi recording the maximum “fog holes” during winter, a study that analysed 17 years of satellite data from NASA shows.

The paper ‘Urban Heat Island over Delhi punches Holes in Widespread Fog in the Indo-Gangetic Plains’ by the Indian Institute of Technology Bombay (IIT-B) and University of Petroleum and Energy Studies, Dehradun, was published in the journal Geophysical Research Letters by the American Geophysical Union on January 8.

The researchers said this is the first study that shows the direct impact and evidence of urban hotspots in fog globally, and specifically over Delhi.

The two-member team found more than 90 occurrences of fog holes in Delhi, where the incidence of fog has dipped by over 50% as compared to its rural surroundings.

The area covers more than 700 square kilometres making it the largest in terms of frequency and extent as compared to the Po Valley in Italy, north China plains and California’s Central Valley, which experience similar phenomena.

With land surface temperatures over Delhi recorded at around four to five degrees Celsius higher compared to its rural surroundings, researchers said the formation of urban heat islands burns the base of the fog layer during mid-morning.

The urban heat island is a phenomenon when the heat gets trapped near the earth’s surface as a result of a decline in green cover, rapid urbanisation, energy-intensive activities, and concrete structures. For instance, vegetation cover in rural areas surrounding Delhi is more than 65% compared to the city.

“Relative humidity, which should be about 95%, is a key factor for fog formation. When surface temperatures increase due to urban heat island effect, there is a decrease in relative humidity which is not conducive to form fog droplets,” lead investigator and senior physical scientist at Washington’s Environmental Defense Fund, Ritesh Gautam, said.

“High humidity levels are important since the air has to be saturated enough with water vapour so that it can condense on pollutants or particles in the air to form fog near the ground,” Gautam, a former IITB professor, added.

From 2000 till 2016, the city recorded fog holes for 55 days. Though fog occurrence frequency has increased by 20% in the IGP from 2000 till 2016, there was no increase in fog over Delhi because of the fog hole phenomena during the same period. The team also found a fourfold decrease in the thickness of the fog around the edges of the hole over Delhi as compared to its surrounding.

Across the IGP that runs across 1,800km, there was a 17% to 36% decline in fog cover across Amritsar, Jalandhar, Patiala, Ludhiana and Lahore. These five cities had 24 to 32 days of fog holes.

Fog is an important climatic feature as it affects local vegetation and weather and fog mixed with pollutants create problems.

“Our study suggests that links between urbanisation and fog dynamics and its frequency should be assessed to better understand the relationships between fog, air pollution and urbanisation, and help advance the development of fog forecasting capabilities,” said Gautam.

Increasing urbanisation may make fog rarer, but there could be some benefits from the change.

“We have a situation where fog is getting amplified by air pollution but urbanisation is decreasing fog. As more urban cities come up, we won’t be able to see the fog but will see holes. However, the fog over Delhi and other places is so notorious that may be urbanisation is helping to keep the polluted fog intensity over Delhi low,” said Gautam.

The findings from the study are important since dense and polluted winter fog in the IGP — with a population of more than 900 million — envelopes north India, Pakistan, Nepal and Bangladesh every year from December to January severely affecting air quality and disrupting air, rail and road traffic.

Madhavan N Rajeevan, the secretary at the Union ministry of earth sciences, said the study will be very useful in understanding the process of why fog occurs and ultimately to predict its occurrence.

“Though we know the basic principles and processes by which fog occurs, its prediction is a very tricky issue. Fog occurrence may vary depending on local conditions such as different topographies or urban heat islands that play an important role in its formation,” Rajeevan said.